1. Field of the Invention
This invention relates to the conversion of coal and similar carbonaceous solids in the presence of alkali metal-containing catalysts and is particularly concerned with the recovery of alkali metal constituents from spent solids produced during coal gasification and similar operations and their reuse as constituents of the alkali metal-containing catalysts.
2. Description of the Prior Art
Potassium carbonate, cesium carbonate and other alkali metal compounds have been recognized as useful catalysts for the gasification of coal and similar carbonaceous solids. The use of such compounds in coal liquefaction, coal carbonization, coal combustion and related processes has been proposed. To secure the higher reaction rates made possible by the presence of the alkali metal compounds it has been suggested that bituminous coal, subbituminous coal, lignite, petroleum coke, oil shale, organic wastes and similar carbonaceous materials be mixed or impregnated with potassium, cesium, sodium or lithium compounds, alone or in combination with other metallic constituents, before such materials are reacted with steam, hydrogen, oxygen or other agents at elevated temperatures to produce gaseous and/or liquid effluents. Studies have shown that a wide variety of different alkali metal compositions can be used for this purpose, including both organic and inorganic salts, oxides, hydroxides and the like. In general the above-described studies indicate that cesium compounds are the most effective gasification catalysts followed by potassium, sodium and lithium compounds in that order. Because of the relatively high cost of cesium compounds and the low effectiveness of lithium compounds, most of the experimental work performed in this area in the past has been directed toward the use of compounds of potassium and sodium. This work has shown that the potassium compounds are substantially more effective than the corresponding sodium compounds. Attention has therefore been focused on the use of potassium carbonate.
Coal gasification processes and similar operations carried out in the presence of alkali metal compounds at high temperatures generally result in the formation of chars and alkali metal residues. The chars normally include unconverted carbonaceous constituents of the coal or other feed material and various inorganic constituents generally referred to as ash. It is generally advisable to withdraw a portion of the char from the reaction zone during gasification and similar operations in order to eliminate the ash and prevent it from building up within the reaction zone or other vessels in the system. Elutriation methods and other techniques for separating char particles of relatively high ash content and returning particles of relatively low ash content to the reaction zone in order to improve the utilization of carbon in such processes has been suggested. In gasification and other processes referred to above that utilize alkali metal-containing catalysts, the cost of the alkali metal constituents is a significant factor in determining the overall cost of the process. In order to maintain catalyst cost at reasonable levels, it is essential that the alkali metal constituents be recovered and reused. There have been proposals for the recovery of alkali metal constituents by leaching as they are withdrawn from the reaction zone with char during operations of the type referred to above. Studies indicate that these constituents are generally present in part as carbonates and other water soluble compounds which can be recovered by water washing. Experience has shown that only a portion of the potassium carbonate or other alkali metal constituents is normally recovered and that substantial quantities of makeup alkali metal compounds are therefore required. This adds appreciably to the cost of such operations.
It has recently been found that increased amounts of alkali metal constituents can be effectively recovered in the form of alkali metal carbonates from the char particles by treating them with calcium hydroxide in the presence of water at elevated temperatures and pressures and thereafter contacting the resultant aqueous solution with carbon dioxide to precipitate any aluminum in solution and to form alkali metal carbonates. The calcium ions from the calcium hydroxide evidently react with alkali metal aluminosilicates and other insoluble alkali metal compounds in the char particles thereby liberating alkali metal constituents which dissolve into solution. The resultant solution is then carbonated and recycled to the reaction zone where the resulting alkali metal carbonates are reused as at least a portion of the alkali metal-containing catalyst. Although this process permits additional recovery of alkali metal constituents in the form of carbonates, a certain amount of expensive, catalytically active makeup alkali metal compounds such as carbonates is still required along with a carbon dioxide stripping step. Moreover, undesirable and poorly catalytic metal sulfates, which are extracted from the char during the treatment step, tend to buildup in the recycle solution.